Comp.hs

{-# LANGUAGE AllowAmbiguousTypes, ConstraintKinds, RankNTypes, ScopedTypeVariables #-}
module CompTest where

import Generics.SOP
import Generics.SOP.Constraint

-- hcmap :: (HAp h, AllN (Prod h) c xs) => proxy c -> (forall a . c a => f a -> f' a) -> h f xs -> h f' xs
-- hcollapse :: (HCollapse h, SListIN h xs) => h (K a) xs -> CollapseTo h a

hcmapQ ::
     forall h c b xs proxy .
     (HAp h, AllN (Prod h) c xs, HCollapse h, SListIN h xs)
  => proxy c -> (forall a . c a => a -> b) -> h I xs -> CollapseTo h b
hcmapQ p f x = hcollapse (hcmap p g x :: h (K b) xs)
  where
    g :: forall a xs . c a => I a -> K b a
    g = K . f . unI